1. Field of the Invention
The present invention relates to an interconnection structure of semiconductor devices, and more particularly to the interconnection structure of a semiconductor device having each of multilayer interconnection layers connected through a connection hole, and a method of manufacturing thereof.
2. Description of the Background Art
Aluminum films and aluminum alloy films of low resistivity are widely used as the interconnection of semiconductor devices. Recently, interconnections of multilayer structure having refractory metals such as tungsten (W), titanium nitride (TiN), molybdenum silicide (MoSi) formed on aluminum films and aluminum alloy films are used for the purpose of improving tolerance for stress migration and electromigration. Stress migration is a phenomenon where the interconnection is discontinued due to film stress of interlayer insulting films and the like formed on the interconnection. Electromigration is a phenomenon where metal atoms migrate under high current density to generate local voids, resulting in increase in resistance and disconnection in the interconnection.
Among the above-mentioned refractory metals, titanium nitride is widely used because of its low light reflectivity. The reason why lower light reflectivity is preferable will be explained with reference to FIGS. 22-25.
Referring to FIG. 22, a field oxide film 3 and a silicon oxide film 5 are formed on a silicon substrate 1. An aluminum interconnection film 7 is formed on silicon oxide film 5. A refractory metal film 9 having relatively high light reflectivity is formed on aluminum interconnection film 7. A silicon oxide film 11 is formed on refractory metal film 9. A resist 13 is formed on silicon oxide film 11.
It is necessary to form a through hole in silicon oxide film 11 in order to electrically connect aluminum interconnection film 7 and another aluminum interconnection film than will be formed later on silicon oxide film 11. Resist 13 is exposed using a mask 15 for the purpose of forming this through hole. Reference number 17 denotes a light transmission blocking portion for blocking the transmission of light. Reference number 19 denotes a light transmission portion that transmits light. Because a through hole is to be formed over a step 6 in aluminum interconnection film 7, the portion of resist 3 above step 6 of aluminum interconnection film 7 is exposed. A portion of light reaches refractory metal film 9 since resist 13 and silicon oxide film 11 have the nature of transmitting light. Light reaches the area in refractory metal 9 comprising a step to be reflected irregularly. This light of irregular reflection exposes resist 13 beneath light transmission blocking portion 17.
FIG. 23 shows the state after exposure, where 21 is the exposed portion of resist 13. Because refractory metal film 9 has high light reflectivity, not only the portion of resist 13 beneath light transmission portion 19, but also some portion of resist 13 beneath light transmission blocking portion 17 is exposed.
Referring to FIG. 24, the exposed portion of resist 13 is removed. The remaining resist 13 is used as a mask to etch silicon oxide film 11 for forming a through hole 23.
Referring to FIG. 25, resist 13 is removed and an aluminum interconnection film 25 is formed on silicon oxide film 11. Aluminum interconnection film 25 is applied the predetermined patterning. This completes the electrical connection between aluminum interconnection film 25 and aluminum interconnection film 7. Because the exposed portion of the resist has been enlarged due to irregular reflection of light, the dimension of through hole 23 is not the designed dimension W1, but dimension W2. Dimension W3 of aluminum interconnection film 25 is made larger than W1 taking into consideration the offset of the mask. However, aluminum interconnection film 7 is also etched at the time of patterning aluminum interconnection film 25 since the dimension of the through hole has become W2.
This explains why titanium nitride of low light reflectivity is widely used as refractory metal film 9. Because the light reflectivity of titanium nitride is low, the portion of resist 13 beneath light transmission blocking portion 17 will not be exposed, as in FIG. 23.
The technique of forming titanium nitride on an aluminum interconnection is disclosed in Japanese Patent Laying-Open No. 63-289935 (1988).
It has been described above that high light reflectivity of refractory metal film 9 is responsible for the dimension of through hole 13 to become larger than the design value. The same thing can be said in patterning the area in aluminum interconnection film 7 comprising the step on which refractory metal film 9 is formed. The dimension of aluminum interconnection film 7 at the step differs from the design value.
The method of electrically connecting a lower layer aluminum interconnection film and an upper layer aluminum interconnection film using titanium nitride as the refractory method will be explained hereinafter with reference to FIGS. 17-21.
Referring to FIG. 17, impurity regions 43 and 45having a distance therebetween are formed in the proximity of the main surface of a silicon substrate 27. A gate electrode 47 is formed through a gate oxide layer (not shown) on the main surface of silicon substrate 27 between impurity regions 43 and 45. Gate electrode 47 has a multilayer structure formed of a polysilicon film 39 and a tungsten silicide film 37. Reference number 41 denotes a sidewall insulating film. Gate electrode 47, impurity regions 43, 45 and silicon substrate 27 implement a MOS (Metal Oxide Semiconductor) field effect transistor.
At the main surface of silicon substrate 27, a field oxide film 29 is formed surrounding this MOS field effect transistor. A silicon oxide film 31 is formed over field oxide film 29 and gate electrode 47. An aluminum interconnection film 49 formed of an aluminum alloy film 33 and a titanium nitride film 35 is provided on silicon oxide film 31.
Referring to FIG. 18, a silicon oxide film 51, and a resist 53 are formed in sequence all over the main surface of silicon substrate 27. In forming silicon oxide film 51, a phenomenon called side hillock occurs when silicon is included in aluminum alloy film 33. This hillock phenomenon is seen in FIG. 18 where hillocks 63 are generated in the sidewalls of aluminum alloy film 33. Side hillocks 63 are generated due to heat occurring in the formation of silicon oxide film 51. The generation of side hillock 63 causes electrical connection between interconnections which should actually be separated, resulting in decrease in yield and reliability.
Resist 53 is exposed to remove a desired portion of resist 53. Then, using resist 53 as a mask, silicon oxide film 51 is subjected to reactive ion etching using CHF.sub.3 gas or CF.sub.4 type gas to form a through hole 55, as shown in FIG. 19.
Since the difference of the etching speeds of the titanium nitride film and the silicon oxide film is small, titanium nitride film 35 is removed by this etching to expose the surface of aluminum alloy film 33. If aluminum alloy film 33 is exposed, the surface layer of aluminum alloy film 33 will be etched. The etched Al and the etching gas are reacted to form a residue 59 at the sidewall of through hole 55. Residue 59 is a compound of Al, F and C.
Referring to FIG. 20, resist 53 is removed by ashing, but residue 59 remains due to its low volatility. It is therefore necessary to remove residue 59 by carrying out wet process of rinsing and with an organic type releasing solution and the like. A denatured layer 57 is generated during this process if aluminum alloy film 33 is exposed due to the high reaction of aluminum alloy. There are some cases where residue 59 is not completely removed so that residue 59 and denatured layer 57 exist at the same time.
Referring to FIG. 21, an aluminum interconnection film 61 is formed on silicon oxide film 51 to be patterned.
If there is residue 59 left in the sidewall of through hole 55, aluminum may not be introduced adequately into through hole 55, leading to a possibility that the electrical connection between aluminum interconnection films 49 and 61 is defective. This will degrade the yield and reliability of the semiconductor device.
There is also a problem that the electrical connection between aluminum interconnection films 61 and 49 may be defective if the amount of denatured layer 57 formed is appreciable since denatured layer 57 is an insulator. This will also degrade the yield and reliability of the semiconductor device.
An approach to increase the film thickness of titanium nitride 35 may be considered for preventing exposure of aluminum alloy film 33 by etching. However, this approach is contrary to obtain the lowest light reflectivity, since the reflectivity is at its minimum only at a certain thickness. The thickness of titanium nitride film 35 cannot be increased also from the standpoint of planarity.
Formation of a tungsten film on the titanium nitride film can prevent the titanium nitride film from being etched and removed even if it is thin. This is because the etching selectivity ratio of tungsten film to silicon oxide film is great. This art is disclosed in Japanese Patent Laying-Open No. 62-132359 (1987). However, the exposure portion of the resist will be enlarged to result in a through hole larger than the design value, as described above, since tungsten has a high light reflectivity.